We have identified a new mitochondrial enzyme system that synthesizes the glutathione thiolester of citric (and of (minus)-erythrofluorocitric acid) by a multienzyme system. Components of this system were isolated on a micro-scale: a. the synthetase containing mixed disulfide of glutathione with a specific protein (58 kd. mass); b. a thiol-rich protein, that oxidized to a disulfide simultaneously with the thiolester synthesis (68 kd. mass). This is the first instance that a protein thiol oxdiation is shown to be coupled to the synthesis of an energy-rich bond (thiolester) in mitochondria. The purpose of work is to integrate this new enzyme system in the citrate transport apparatus of mitochondria. The work is related to bioenergetics and has relevance to metabolic diseases.